Problem: On the first 5 exams of her language class, Gabriela got an average score of 81. What does she need on the next exam to have an overall average of 84?
Let her score on the next exam be $x$ The sum of all of her scores is then $5 \cdot 81 + x$ The same sum must also be equal to $6 \cdot 84$ Solve: $x = 6 \cdot 84 - 5 \cdot 81 = 99$.